A filter for removing particulate matter from an exhaust gas emitted to the atmosphere is known in the art. For example, Japanese Patent Laid-Open No. 9-94434 discloses a wall flow type waste gas purifying filter. According to the invention disclosed in Japanese Patent Laid-Open No. 9-94434, the particulate matter is trapped in the pores formed in a filter substrate when the exhaust gas passes therethrough, and the particulate matter is oxidized by a catalyst carried inside of the pores.
Japanese Patent Publication No. 07-106290 discloses a catalyst mixture consisting essentially of a platinum group metal and an alkaline earth metal oxide which is used in the filter of this kind. According to the invention disclosed in Japanese Patent Publication No. 07-106290, the catalyst mixture is carried on the filter element walls by a deposition method. Also, Japanese Patent Laid-Open No. 2001-271634 discloses an exhaust emission control device, in which a carried catalyst contains an active oxygen discharging agent.
Basically, a filter for removing the particulate matter is adapted to trap the particulate matter contained in the exhaust gas on an outer surface of a porous filter wall (or filter substrate) and an inner surface of a pore, when the exhaust gas passes through the filter wall. According to the inventions disclosed in the above-mentioned publications, the trapped particulate matter is removed by oxidizing (or combusting) the trapped particulate matter aggressively through the catalyst.
The oxidation reaction is produced by active oxygen produced by the catalyst taught by Japanese Patent Laid-Open No. 9-94434 or Japanese Patent Publication No. 07-106290, or by active oxygen emitted in a reducing atmosphere from the active oxygen discharging agent contained in the catalyst taught by Japanese Patent Laid-Open No. 2001-271634. However, the activation region is limited within the range of several nm from the place where the active oxygen is produced.
Meanwhile, the catalyst is carried on the filter substrate by a various method e.g., by a deposition method taught by Japanese Patent Publication No. 07-106290. However, since the catalyst is a particulate material, the catalyst may not always be carried densely on the surface of the filter substrate. For example, an uncoated area 4 where the surface of the filter substrate 3 is exposed is created between catalyst particles 1 as illustrated in FIG. 5 (B). In case a distance of the uncoated area 4 is several μm to about 100 μm, a particulate matter 5 may adhere to (or is trapped on) the uncoated area 4, and the particulate matter 5 may be deposited thereon without being oxidized by the active oxygen.
As a matter of course the particulate matter 5 thus deposited does not comprises a catalytic element on its surface. Moreover, since the particulate matter 5 thus deposited is distant from the peripheral catalyst particles 1, there still remains the uncoated area 4. Therefore, the subsequent particulate matter 5 will adhere thereto and will be deposited without being oxidized. FIG. 5 (A) schematically shows such sedimentation. As shown in FIG. 5 (A), the particulate matter 5 adheres to and deposited on the uncoated area 4 grows gradually to form a bridge beam like shape. Eventually, adjoining upper end portions of the bridge beams will be connected with each other. As a result, the particulate matter 5 forms a porous membrane 6 on an outer surface of the filter substrate 3.
If the porous membrane 6 is formed on the outer surface of the filter substrate 3, the particulate matter 5 in an exhaust gas is trapped by the porous membrane 6 when the exhaust gas passes through the porous membrane 6. This means that, the particulate matter 5 cannot reach the filter substrate 3 carrying the catalyst thereon. That is, although it is possible to trap the particulate matter 5, the particulate matter 5 cannot be oxidized to be removed. As a result, an amount of the trapped particulate matter 5 may be accumulated excessive.
Here, it is assumed that the uncoated area without having the catalyst also exists on an inner surface of a pore 7 in the filter substrate 3; however, above-explained sedimentation is not especially confirmed. This is because the particulate matter 5 may be moved within the pore 7 to be oxidized due to the fact that: the spatial configuration in the pore 7 is rather complicated; the flow rate of the exhaust gas is high; and the flow force of the exhaust gas is likely to act on the particulate matter 5 to move the particle matter 5 along the inner surface of the pore 7.